1. Field of the Invention
This invention relates to a rotor with salient poles for electric rotating machines such as a hydraulic turbine generator, or more in particular to a construction of the shield plates for sealing the gaps between the poles.
2. Description of the Prior Art
The hydraulic turbine generator has recently increased remarkably both in capacity and speed. This is especially the case with the high head pumping-up power plant. In the hydraulic turbine generator of a high capacity and speed, the peripheral speed of the rotor is high, and the air friction loss thereof is proportional to the cube of the peripheral speed, resulting in an increased power loss and a reduced efficiency. For this reason, attempts have been made to reduce the air friction loss by providing the rotor with a substantially cylindrical surface.
Examples of such attempts are disclosed in Japanese Utility Model Application No. 131642/74 (Laid-open No. 58,802/76) and U.S. Pat. No. 3,157,806. In these prior art machines, as shown in FIG. 1, the rotor comprises a rotor rim 1 fixed on a rotor shaft (not shown), salient poles 2 with field coils 3 mounted to the rotor rim and non-magnetic shield plates 4 each supported between the heads of adjacent poles 2 so that the rotor presents a substantially cylindrical configuration, thereby reducing its air friction loss.
In this construction, however, when the rotor is increased in diameter and speed for increasing its capacity, it naturally results in an increased centrifugal force exerted on the shield plates 4. Therefore, the shield plates 4 are required to be made thicker.
Let the thickness of the seal plate 4 be t, the force exerted on the shield plate 4 be F, and the bending stress acting on the shield plate 4 be .sigma..sub.b. Since the modulus of section of the shield plate 4 is proportional to the square of t, EQU .sigma..sub.b .varies.(F/t.sup.2) (1)
While, the mass of the shield plate is proportional to the thickness t, the centrifugal force F is expressed as EQU F.varies.D.omega..sup.2 t (2)
where D is the diameter and .omega. the angular velocity of the rotor. From the expressions (1) and (2), EQU .sigma..sub.b .varies.(D.omega..sup.2 /t) (3)
When the angular velocity of the rotor is increased, the bending stress .sigma..sub.b is also increased and hence the shield plate must be strengthened so as to be unbroken against the increased bending stress. Since the strength of the material for the shield plate is limited, it is required generally to increase the thickness of the shield plate.
In order to increase the thickness of the shield plate 4, the heads or eaves of the poles 2, where the shield plates are mounted, must be made thicker as shown in FIG. 2. This increases the size of the salient-pole, resulting in increasing the outer diameter and weight of the rotor.
Further, the magnetic lines of force generated by the current flowing in the stator coil 6 inserted into the slots of the stator core 5 are distributed as shown by dotted lines .PHI. in FIG. 2. Since the magnetic pole is of a ferromagnetic material, the magnetic lines of force 100 are concentrated at the eaves of the pole 2 and run substantially parallel with the shield plate 4 in the neighbourhood of the eaves. As a result, in the case where the shield plate 4 is thick and made of an electrically conductive metallic sheet as generally used, an eddy current Ie as shown in FIG. 3 flows. This causes not only the shield plates 4 to heat, but also the power loss to increase.